The answer is that RA Guard configured with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, which inadvertently blocks OSPFv3 hello packets sent to the multicast address FF02::5. This occurs because the RA Guard policy, when set to host mode, treats the connected router as an end device and filters out non-essential IPv6 traffic, including the OSPFv3 hellos required for adjacency formation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how security features like RA Guard can interfere with routing protocols—a common trap is assuming RA Guard only blocks Router Advertisements, when in fact the device-role host parameter imposes a broader filter. Remember the memory tip: "Host mode halts hellos"—if a router port is misclassified as a host, OSPFv3 adjacencies will fail because the switch drops the protocol traffic it deems unnecessary for an end device.
300-410 IPv6 First Hop Security Practice Question
This 300-410 practice question tests your understanding of ipv6 first hop security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer configures IPv6 RA Guard on a switch port connected to a router running OSPFv3. Unexpectedly, OSPFv3 neighbor adjacencies fail to form on that link. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
RA Guard drops all Router Advertisement messages, but OSPFv3 uses IPv6 multicast address FF02::5 for hello packets. However, RA Guard can also be configured to drop all ICMPv6 messages with type 134 (RA), but the default RA Guard policy blocks all RAs. If the router sends RAs (even if not needed), the port might be err-disabled or the RAs are dropped, but the issue is that OSPFv3 hellos are not affected. The real edge case: RA Guard with 'device-role host' blocks all IPv6 traffic from the port except ND and DHCP, which can include OSPFv3 if the policy is too restrictive. The most common misconfiguration is that RA Guard is applied with 'device-role switch' which expects the port to be a switch, but if the port is actually a router, the router's OSPFv3 hellos are dropped because RA Guard treats the router as a host and drops non-ND traffic.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
Why this is correct
RA Guard host role restricts traffic to ND and DHCP only, blocking OSPFv3.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
OSPFv3 uses multicast address FF02::5 which is filtered by RA Guard by default.
Why it's wrong here
RA Guard does not filter OSPFv3 multicast unless configured to do so.
✗
RA Guard changes the MAC address of the router, causing OSPFv3 neighbor to be unreachable.
Why it's wrong here
RA Guard does not modify MAC addresses.
✗
The router must send Router Advertisements for OSPFv3 to work, and RA Guard blocks them.
Why it's wrong here
OSPFv3 does not require RAs.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos. — RA Guard drops all Router Advertisement messages, but OSPFv3 uses IPv6 multicast address FF02::5 for hello packets. However, RA Guard can also be configured to drop all ICMPv6 messages with type 134 (RA), but the default RA Guard policy blocks all RAs. If the router sends RAs (even if not needed), the port might be err-disabled or the RAs are dropped, but the issue is that OSPFv3 hellos are not affected. The real edge case: RA Guard with 'device-role host' blocks all IPv6 traffic from the port except ND and DHCP, which can include OSPFv3 if the policy is too restrictive. The most common misconfiguration is that RA Guard is applied with 'device-role switch' which expects the port to be a switch, but if the port is actually a router, the router's OSPFv3 hellos are dropped because RA Guard treats the router as a host and drops non-ND traffic.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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